Information for Referring Veterinarians

WHY SHOULD I REFER FOR STANDING EQUINE MRI?

Standing MRI offers a unique insight into equine lameness, identifying the specific cause in over 90% of cases. With a clear diagnosis, the client can be offered informed advice not only with regard to treatment but also to prognosis and the length of time any recovery is likely to take.

The most common approach to equine lameness is to conduct a thorough examination, including observation of the response to local anaesthetic 'nerve blocks', x-ray and ultrasound examinations. Based on the case history, these observations, and other knowledge and skills available the responsible veterinarian will recommend rest or treatment.

In about 70% of cases, such treatment will not result in a satisfactory outcome, and the patient will return for a further examination. Without MRI, further diagnosis and treatment efforts are still only successful about 30% of the time. Cases frequently cycle around the loop of attempted diagnosis and treatment many times. Repeated failures frustrate client and vet alike, increase the client's costs, and potentially exacerbate the condition.

While any one loop around the cycle might appear to be the low-cost option, the total cost may well exceed the cost of a definitive MRI diagnosis.

Don't consider MRI as a last resort, get the answers that you need sooner rather than later.

Early - MRI can be undertaken as soon as nerve blocks confirm the location of lameness.

Safe - The Hallmarq equine MRI system offers the choice of standing MRI, giving you the quality images that you need with the horse under light sedation avoiding the risks of general anaesthesia.

Accurate - Each scan generates between 300-500 images per foot, offering a 90% likelihood of conclusive diagnosis, helping to target treatment and obtain a more satisfactory outcome.

About half of all lameness cases are due to problems located to the foot, frequently toward the heel area. This region contains the navicular bone and, as this is the only structure visible on x-ray, these lamenesses were for a long time called "navicular disease" or "navicular syndrome". While some disease is indeed due to changes in the bone other structures such as the navicular bursa, coffin joint, tendons and ligaments can also be the source of pain. As the hard hoof wall prevents diagnosis of soft tissue disease by ultrasound, and nuclear scintigraphy has very limited resolution, only MRI can give a comprehensive image of both bone and soft tissue. MRI, picturing the inside of the foot as "slices" through the tissue in different planes, can accurately visualise all the key structures. With many thousands of horses now having undergone a standing MRI examination, experienced radiologists and other experts can call upon their extensive experience to give a diagnosis with more confidence than ever before.

The image above shows a severe lesion of the collateral ligament of the DIP joint (hyperintense region toward the top right. Such lesions were rarely identified as the cause of foot lameness before MRI was available, as the majority of the ligament is not accessible to ultrasound and it is not visible on x-ray.

Regional anaesthesia is also now understood to be less specific than traditionally taught, and even more, proximal regions such as this collateral ligament can still block out to the palmar/plantar digital nerve block conventionally taken as diagnostic of the palmar/plantar region.

IMPACT OF DIAGNOSIS ON TREATMENT

A wide range of treatments are now available, for conditions that fall under the "navicular syndrome" umbrella and others.

Conservative: box rest, programmed exercise, NSAIDs

Non-invasive: remedial farriery, shockwave

Medication: NSAIDS, steroids, bisphosphonates

Biologicals: stem cells, PRP

Surgery: bursoscopy, defibrillation, neurectomy, fracture fixation

Knowing the cause of the lameness allows the veterinarian to recommend only treatments and therapies that might reach the affected area and improve the horse's condition. A clear diagnosis saves the owner time and money and allows better plans to be made for the animal's future, while the veterinarian is secure in the knowledge that the real injury is being treated.

AVOIDING THE RISK OF GENERAL ANAESTHESIA

General anaesthesia is familiar territory to equine veterinarians, especially in the context of surgery, and is routinely used where the benefit exceeds the risk. But even in a horse without complicating factors published data* suggest general anaesthesia carries a 1% risk of fatality, and a higher risk of myopathy or other complications. Increasingly avoiding anaesthesia is seen as a worthy aim, and new techniques for standing surgery, for example in the pastern, the head and the back are becoming more common.

For diagnosis alone the risk due to general anaesthesia is considered unacceptable by many horse owners, and in certain cases eg racehorses in training it cannot be used at all. With standing MRI there is no need for general anaesthesia. The horse remains conscious and standing throughout the entire procedure. Indeed even the sedation used is lighter than for many veterinary procedures because the horse experiences no pain and no interventions, simply standing near the magnet for an hour or so. Recovery is swift, and most horses can go home on the same day. Veterinarians benefit from the reduced risk, cost and staff time, and clients benefit from the reduced worry and the faster return of their horse.

MRI is a powerful diagnostic tool, used extensively in human and veterinary medicine for orthopaedic and neurological diagnosis. In the horse, it is commonly used for certain forms of lameness originating in the foot up to and including the knee (carpus) and hock (tarsus).

When tissue is placed in a strong magnetic field and a short pulse of radio waves is applied, a weak signal echoes back and is used to create an image. The tissue examined must be completely inside an extremely strong magnet. Magnets are of two types: high field electromagnets are tubular in shape and are cooled with liquid helium; low field magnets are U-shaped or a sandwich with a gap between two flat plates. The Hallmarq standing MRI is U-shaped so that the standing horse can walk in an out of the magnet safely, allowing access to the distal limbs up to the carpus and distal tarsus. Tubular magnets can, under general anaesthesia, be used to scan distal limbs and, for some horses, other regions (heads, stifle).

IS MRI SAFE?

No ionising radiation is used (as in X-rays) and MRI has no known hazardous biological effects. The field can though activate a switch inside a cardiac pacemaker, and metal objects can be attracted with hazardous force, so public access close to the magnet must be restricted.

Because MRI transmits and receives radio signals, scanning must take place in a metal screened room which prevents interference from external radio waves. The room is also convenient for restricting unauthorised access, though as it does not affect the static magnet field there may be further restrictions (e.g. regarding sensitive equipment or heavy vehicles) in the nearby area.

WHAT DO MR IMAGES LOOK LIKE?

MR images appear as black-and-white slices through the tissue. The position and orientation of the slice are chosen by the scanner operator.

Vertical slice through the midline of the hoof, pastern, and fetlock from front to back

Vertical slice through the fetlock joint from side to side

Two different slices through the fetlock joint from front to back

Vertical slice through the hoof (from side to side) showing the coffin and pedal bone

Horizontal slice through the knee (carpus)

Horizontal slice through the top of the cannon bone showing bone, tendons, and ligaments

WHAT ARE MRI SEQUENCES?

The exact appearance of an MR image is determined by the timing of the various radiofrequency and magnetic field gradient pulses used to collect the data, collectively known as the imaging sequence. Different sequences generate images that show fat (eg in bone) and water (eg oedema) at different brightnesses.

A full range of sequences is available using the Hallmarq system, including T1 and T2 weighted gradient echo and fast spin echo images, and fat suppressed STIR images. All sequence types are available with motion detection and correction to assist scanning the higher joints.

Clinical image interpretation relies upon an expert knowledge of anatomy and the MRI appearance of normal and abnormal tissue with different sequences.

T1 weighted - Optimised for visualisation of anatomical structures

T1 weighted sagittal section through an equine foot. The navicular bone is markedly abnormal, appearing dark where normal bone appears more like the digital phalanges in this image.